The present invention relates to a vertical field-type MRI apparatus for forming magnetic resonance images, including at least one field generating superconducting coil system for producing a substantially homogeneous magnetic field in an imaging volume of the apparatus, which coil system includes an outer coil that is situated in a flat outer coil plane and a supplementary coil that is situated within the outer coil, and at least one gradient coil system for producing a magnetic gradient field in the imaging volume of the apparatus. The gradient coil system includes a flat main gradient coil and a shielding coil.
An apparatus of this kind is known from U.S. Pat. No. 5,939,962. The homogeneous magnetic field required for magnetic resonance imaging in such a vertical field type apparatus is usually generated by two oppositely situated magnetic poles wherebetween the patient to be examined can be arranged. Generally speaking, said magnetic field then has a vertical direction. Apparatus of this kind offer the advantage that the patient keeps a comparatively broad view of the surroundings when arranged in such an apparatus, so that sensations of claustrophobia occur less frequently.
An iron circuit that is capable of transporting the complete flux through the system becomes very heavy in the case of magnet systems having a field strength beyond approximately 0.5 T. A sensible alternative in that case is to omit the iron circuit completely and to construct the magnet system as an actively shielded air coil system. In that case there are no poles in the sense of iron structures that bound the space of the magnet system that is accessible to the patient, but the surfaces of the magnet system that bound the patient space will also be referred to hereinafter as xe2x80x9cpolesxe2x80x9d for the sake of simplicity. For field strengths beyond 0.5 T the coils must be constructed so as to be superconducting. They are kept at the operating temperature in a cryostat. The xe2x80x9cpolesxe2x80x9d are then formed by the outer wall of the vacuum envelope of the cryostat.
The cited United States patent discloses a superconducting coil system which consists of a round outer coil (a so-called xe2x80x9cside coilxe2x80x9d that is denoted by the reference 12a therein), a round supplementary coil (referred to therein as xe2x80x9cthe fourth coilxe2x80x9d which bears the reference 12d), and a number of further coils (referred to as xe2x80x9cthe second and the third coilsxe2x80x9d bearing the references 12b and 12c therein). The homogeneous field in the imaging volume is generated mainly by the first two coils 12a and 12d and the other coils mentioned superpose a further homogenizing field thereon.
As is generally known, and also described in the cited United States patent, for this type of apparatus the aim is to arrange the field generating coil in the upper magnetic pole at an as short as possible distance from the field generating coil in the lower magnetic pole. This aim stems from the fact that the production costs of such a system increase by approximately a power of five of said pole distance, so that it is advantageous to keep this distance as small as possible. Because of this aim, the outer coils in the known apparatus are arranged practically directly against the boundary of the freely accessible space between the magnetic poles.
When the outer coils are mounted in that manner, the gradient coils in the known apparatus must extend to practically the diameter of the associated outer coils because of the necessary linearity of the gradient field in the imaging volume. Consequently, room for the supplementary coil (also having a voluminous and heavy construction so as to achieve the required homogeneous field) can be found only above the upper gradient coil and below the lower gradient coil. Consequently, the construction of this already bulky and heavy coil must be even larger; however, the outer coil must then also become larger. Moreover, in the case of actively shielded magnetic coils the shielding coils must then also become larger. The ultimate effect of the foregoing is that the costs of the apparatus are significantly increased again.
It is an object of the present invention to provide a vertical field-type MRI apparatus in which the distance between the outer coils and between the supplementary coils is as small as possible. To achieve this object and others, in the apparatus in accordance with the invention, the supplementary coil is situated in the outer coil plane, the energizing of the outer coil and the supplementary coil is such that these coils generate magnetic fields of opposite direction, the ratio Da/Do of the diameter Da of the supplementary coil to the diameter Do of the outer coil is between 0.7 and 0.9, the apex of the shielding coil of the gradient coil system is situated at a distance from the main gradient coil which is larger than the distance between the edge of the shielding coil and the main gradient coil, and the gradient coil system is situated at least partly in a space within the supplementary coil.
The invention is based on the recognition of the fact that the minimum distance between the field-generating poles can be realized by imparting a special shape to the shielding coil of the gradient coil system, that is, a shape such that the center or apex of the shielding coil is situated at a distance from the main gradient coil which is larger than the distance from the edge of the shielding coil to the main gradient coil. The shape of the shielding coil is thus substantially conical. As a result of the shape of the gradient coil system, the supplementary coil which is energized in an opposite sense or direction relative to the outer coil can be situated in the same plane (the outer coil plane) as the outer coil. This yields a suitably homogeneous field in the imaging volume if the ratio Da/Do of the diameter Da of the supplementary coil to the diameter Do of the outer coil is between 0.7 and 0.9. A space still remains in the supplementary coil so as to accommodate the gradient coil system thus formed.
Said special shape of the gradient coil system also yields an additional advantage is obtained. This is because it has been found that said system can be proportioned in such a manner that a slit-like space remains between the shielding coil of the system and a corresponding recess in the cryo container of the coil system that generates the homogeneous field. This space can be used to accommodate the so-called shim iron which is used in known manner so as to compensate changes of the magnetic environment, that is, to compensate circumstances degrading the homogeneity. Periodic adaptations of the shim state are necessary and to this end the shim iron is then readily accessible and can be displaced without it being necessary to remove the entire system of gradient coils. This advantage is due to a generally non-angular profile of said special shape of the shielding coil.
The shielding coil in a preferred embodiment of the invention extends across a substantially conical surface. This conical shape is then taken up in said free space within the supplementary coil and the flat main gradient coil is then situated substantially in the outer coil plane.
An advantageous embodiment of the apparatus in accordance with the invention is provided with three further coils, each of which is situated in a respective further coil plane, the outer coil plane being situated between the imaging volume and each of said further coil planes
It has been found that a suitable compromise can thus be achieved between production costs (condition: few and small further coils) and field strength and homogeneity (condition: many and large further coils).
In a further advantageous embodiment in accordance with the invention the three further coils are situated on a conical surface, the apex of the conical surface being directed away from the imaging volume.
Apart from the fact that this configuration very well satisfies the requirements as regards field strength and homogeneity, this embodiment notably offers the advantage that there is created an inner space (that is, a space around the vertical axis of the imaging volume) which, because of its conical shape, is very compatible with a gradient coil having a conical external appearance. This shape of a cavity in the magnetic pole, that is, in the cryo container of the magnetic coils, also has an additional advantage. In normal operating conditions the cooling medium present in a cryo container, that is, liquid helium, has a pressure of approximately 1 bar. In given circumstances, however, this pressure may increase to as much as 3 bar. The helium container is surrounded by a vacuum space which, therefore, lies between the ambient atmosphere and the helium container. In the case of a cavity with square corners, as in the present state of the art, extreme mechanical stresses could occur at said pressures; when a more or less conical cavity is used, such stresses will occur to a much lesser extent because of the gradual shaping of the walls of the container.
A preferred embodiment of the apparatus in accordance with the invention is provided with a second field generating superconducting coil system for producing a substantially homogeneous magnetic field in the imaging volume of the apparatus,
which second coil system includes:
a second outer coil which is situated in a second flat outer coil plane and whose diameter is larger than that of said first outer coil,
a second supplementary coil which is situated within the second outer coil and in the second outer coil plane,
which apparatus also includes a second gradient coil system for producing a magnetic gradient field in the imaging volume of the apparatus, said second gradient coil system including a flat main gradient coil and a shielding coil,
the energizing of the second outer coil and of the second supplementary coil being such that these coils generate magnetic fields of opposite direction,
the ratio Da/Do of the diameter Da of the second supplementary coil to the diameter Do of the second outer coil being between 0.7 and 0.9,
the center of the shielding coil of the second gradient coil system being situated at a distance from the main gradient coil of the second gradient coil system which is larger than the distance between the edge of said shielding coil and the main gradient coil, and
the second gradient coil system being situated at least partly in a space within the second supplementary coil.
It is feasible to provide the coil system for generating the homogeneous field with only one pole surface; in such apparatus a given concession is made in respect of homogeneity and field strength, but it nevertheless remains possible to use the apparatus for given medical purposes. An apparatus of this kind is known, for example, from U.S. Pat. No. 5,917,395. When the apparatus is constructed so as to have two pole surfaces as is more usual, an attractive location of the imaging volume relative to the pole surfaces can be chosen. This offers an advantage in the following circumstances: a given size of the imaging volume is defined in dependence on the amount of space desired for the patient. This size defines the minimum distance between the pole surfaces. It should be possible to make optimum use of this distance for all imaging purposes, notably the imaging of parts of the body that are situated at a low level in the imaging volume, for example the vertebral column which is situated directly above the table top in the case of a patient in the supine position. This table top, of course, should be as thin as possible as otherwise space that could be used for imaging is lost or the pole surfaces have to be situated further apart again. The vertebral column is then situated at the edge of the imaging volume, so that optimum homogeneity is not possible over a great length. As a result of said steps (notably because the lower outer coil has a diameter which is larger than that of the upper coil), the imaging volume can be lowered relative to the pole surfaces, so that the vertebral column fits better in the imaging volume, without it being necessary to enlarge the latter (which is expensive and leads to a high power consumption during operation).
The second shielding coil in a further embodiment of the invention extends across a substantially conical surface. The conical shape is then taken up in said free space within the second supplementary coil and the flat main gradient coil is then situated substantially in the second outer coil plane.
An advantageous embodiment of the apparatus in accordance with the invention is provided with four further coils, each of which is situated in a respective further coil plane, the second outer coil plane being situated between the imaging volume and each of said further coil planes.
It has been found that said number of further coils enables a suitable compromise to be achieved between production costs, field strength and homogeneity in the case of the desired lowering of the imaging volume.
The four further coils in a further embodiment of the invention are situated on a conical surface, the apex of the conical surface being directed away from the imaging volume.
Thus, a space is created within the coil container for the main field, the conical shape of said space being very well compatible with a gradient coil having a conical external appearance.